1. Field of the Invention
The present invention relates to a solid polymer electrolyte membrane type fuel cell, and a fuel cell stack constituted by stacking a plurality of said fuel cell units, and more specifically relates to a technique effective in absorbing increase and decrease in the stacking direction of separators.
2. Description of the Related Art
Fuel cells include a solid polymer electrolyte membrane type fuel cell constituted by providing a pair of electrodes on opposite sides of the solid polymer electrolyte membrane and sandwiching the outside thereof by a pair of separators.
In this fuel cell, a channel for a fuel gas (for example hydrogen) is provided on the whole surface of a separator provided facing one electrode, a channel for an oxidant gas (for example air including oxygen) is provided on the whole surface of a separator provided facing the other electrode, and a channel for a cooling medium is provided on either one of the surface of separators opposite to a surface facing the electrode.
When the fuel gas is supplied to the reaction surface of one electrode, hydrogen is ionized, and moves to the other electrode via the solid polymer electrolyte membrane. Electrons generated during the reaction process are taken out to an external circuit, and are used as direct-current electric energy.
Since the oxidant gas is supplied to the other electrode, the hydrogen ions, the electrons and the oxygen react with each other to thereby generate water.
The surface on the opposite side of the electrode reaction plane of the separator is cooled by the cooling medium flowing between the separators.
Since these reactant gas and cooling medium should flow in respectively independent channels, a sealing technique, which separates each channel, is important.
The portions to be sealed include; the peripheries of communication holes formed penetrating through the separator so as to distribute and supply the reactant gas and the cooling medium to each fuel cell unit in the fuel cell stack, the outer peripheries of membrane electrode assembly formed of the solid polymer electrolyte membrane and a pair of electrodes arranged on opposite sides thereof, the outer peripheries of a coolant passage plane of the separator, and the outer peripheries of front and back faces of the separator. As the sealing material, a elastic and adequately resilient material, for example, an organic rubber, is adopted.
In the case where the fuel cells are stacked in a plurality of numbers to construct the fuel cell stack, and this fuel cell stack is mounted in a vehicle, there may be a case where drops of water splash and the fuel cell becomes wet, depending on the installed position, or dust enters into the gap between separators.
However, the sealing material can prevent such water and dust from entering into the reactant gas channel or into the cooling medium channel.
At the time of stacking the separators, if there is a difference in thickness between the membrane electrode assembly, or if there is bending or distortion in the separator (particularly, in a thin separator made of metal), or if the compressive load applied from the opposite ends of the fuel cell stack is non-uniform, the separators are not stacked parallel with each other, and inclination or warp occurs. Hence, the compression amount of each sealing material becomes unequal, and as a result, sealability deteriorates in sealing material having a small amount of compression.
At the time of stacking the separators, it is also difficult to stack these separators accurately without shifting their relative positions along the electrode reaction plane.
As measures against the above problems, there can be considered a method in which foreign substance is prevented from entering into the gap between the separators, and the separators are stacked parallel with each other, by providing, for example, a picture frame-shaped member made of resin, at the outer edge of the separator.
Techniques similar to this are disclosed in, for example, Japanese Unexamined Patent Application, First Publication Nos. Hei 10-74530, Hei 7-249417 and Sho 61-279069.
However, if the sealing material or the membrane electrode assembly shrinks in the stacking direction of the separators due to deterioration with the lapse of time, or the fuel cell expands or contracts due to the influence of heat or the like, the following problems will occur.
For example, when the protruding height of the sealing material from the separator becomes lower than the protruding height of the picture frame-shaped member, shrinkage of the space between separators is restricted by the picture frame-shaped member. Hence, a gap may be caused between the separator and the sealing material or the membrane electrode assembly, thereby causing a decrease in the power generation performance, and consequently causing a situation where power generation is not possible.
On the other hand, if the space between separators expands due to the influence of heat or the like, a sealing material such as a rubber will be resiliently restored and extend in the stacking direction of the separators, and hence, this sealing material can follow the expansion of the space between separators without separating from the separator, to some extent. A picture frame-shaped member made of resin or the like, however, since this does not expand in the stacking direction of the separators, this cannot accommodate the expansion of the space between separators.
Therefore, a gap occurs between the picture frame-shaped members, and foreign substance may enter there.
Moreover, it is desired to prevent a liquid connection by the cooling medium, which causes an electric current flowing through the cooling medium, and it is also necessary to prevent adjacent separators in the reactant gas channel from being electrically short circuited.
Particularly, in the case of a fuel cell using thin metal separators, since the space between separators is small, it is particularly desired to specially take measures to prevent electrical short circuited from occurring between adjacent separators, taking into account that foreign substance such as dust and carbon particles become mixed in the reactant gas.